What oil and what mix ratio are you using? Too much oil and you'll get excessive amounts of oil at the tailpipe, but as was said above, oil in the exhaust is normal for a 2 stroke engine. About the only exception is those weird guys, I'm one of them, that run the synthetics at very low oil to gas ratios. I'm at 100:1 and see very little oil residue from the exhaust. I'm not recommending you go that route but just food for thought. If you're still in the break in period don't go any higher in oil than 24:1 (disregard the instructions which might tell you 16:1) Better yet go to 32:1 and stay there.
As for cleaning your chrome muffler, I'd use Brake Clean as opposed to steel wool. It will cut through the brown stuff and not damage or scuff the chrome.
Tom